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ELECTRICAL 

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SAFETY DEVICES 


BY 

JAMES G. PEEBLES 

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CHICAGO : 

THE JOSEPH G. BRANCH PUBLISHING COMPANY 

1913 



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Copyright, 1913, by 
Joseph G. Branch 


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MAY 13 1314 


THE HENRY O. SHEPARD CO., PRINTERS, CHICAGO. 


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PREFACE 


The object of the author in preparing this booklet 
is to explain as clearly and simply as possible the 
construction and operation of the principal safety 
devices used in connection with electrical machinery. 
Such devices are of vital importance in the successful 
operation of electrical equipment, and a clear under¬ 
standing of them on the part of the operating engineer 
and the electrician is highly desirable. 

The devices discussed here may be divided into 
two classes: First, those which afford positive protec¬ 
tion against abnormal conditions of current or pres¬ 
sure, such as the various forms of fuses, circuit-break¬ 
ers and lightning arresters. Second, those which tend 
to correct any unusual conditions, such as the differ¬ 
ent forms of regulators. Those of the first class are 
purely protective, those of the second are regulative, 
and thus prevent dangerous conditions from arising. 

There is sometimes a tendency on the part of those 
who have to do with the operation of electrical equip¬ 
ment to regard the^safety device as a necessary evil. 
It is hoped that a careful reading of the following 
pages will make plain the important function of such 
equipment, and induce the man in charge to give it his 
very best attention. Only those protective devices 
which are found in almost every electrical plant are 

iii 



IV 


ELECTRICAL SAFETY DEVICES 


discussed, and for this reason it is not claimed that the 
whole field has been covered. It is believed that if the 
operator of electrical machinery has a clear under¬ 
standing of fuses, circuit-breakers, lightning arresters 
and the different forms of regulators, he will have a 
greater respect for protective devices in general. If 
this is the result, the preparation of these pages will 
have been well worth while. 

The author wishes to extend his thanks to the 
various manufacturers of protective equipment who 
have supplied many of the illustrations used. Ac¬ 
knowledgment has also been made in the text when¬ 
ever possible. 

JAMES C. PEEBLES. 


Chicago, May 19, 1913. 


Electrical Safety Devices 


CHAPTER I. 

In all machinery designed for the generation or 
utilization of power, the importance of automatic safety 
devices has been recognized from the beginning. Thus, 
the advent of the safety valve occurred at practically 
the same time as that of the steam boiler, and the 
governor for controlling the speed of a steam engine 
was developed as an important step in the design of 
the prime mover itself. The same is true in all power 
machinery: safety devices for the protection of the 
equipment from as many sources of injury as possible 
are always used. 

The wide application of electricity to many kinds 
of industry has led to the development of a large 
variety of electrical safety devices, designed for the 
protection of equipment using this form of energy. 
The chief source of danger to electrical machinery is 
probably to be found in an abnormally large flow of 
current, greater than that for which the equipment 
is designed. This has led to the development of pro¬ 
tective devices which automatically open the circuit 
whenever the current exceeds a certain safe maximum. 

Early experiments with electricity soon disclosed 
the fact that a small wire may be fused by the heat 
generated by an electric current. Thus, if such a 
piece of wire be introduced into an electric circuit, it 
will serve to protect the rest of the circuit when 



2 


ELECTRICAL SAEETY DEVICES 


the current becomes excessive, by being destroyed 
itself. Thus the thermal action of the current can be 
used as a means of protection. Another method of 
opening a circuit is found in the magnetic properties 
of the current, which make possible the design of a 
mechanical device, allowing for the passage of the nor¬ 
mal current but operating instantly when the current 
becomes excessive. The thermal property of the cur¬ 
rent has made possible the development of the fuse as 
a protective device, while the magnetic property has 
been responsible for the mechanical circuit-breaker. 

Fuses. 

When damage occurs to electrical equipment it is 
due in the great majority of cases to overheating. It 
is the thermal action of the current which causes the 
trouble, and it is exactly this same effect upon which 
the fuse depends for its action. Thus the fuse is par¬ 
ticularly well suited for a protecting device, because 
it forms a part of the circuit and partakes of all the 
characteristics of the same. Its temperature rises and 
falls with that of the other conductors in the circuit, 
and it opens promptly when enough heat has been 
generated to become dangerous. 

The first fuses used consisted merely of a small 
piece of copper wire introduced into the circuit, which 
fused when the current became excessive. On account 
of the relatively high melting point of copper, dan¬ 
gerous temperatures were reached when the copper 
fuse melted. Thus the copper fuse was never seriously 
considered. Lead and many of its alloys have com¬ 
paratively low melting points, and hence are more 
adaptable for fuse construction. The method of appli¬ 
cation was simply to connect this piece of lead alloy 
wire into the circuit. To do this various kinds of 
cut-out bases were designed of porcelain or slate, 
which provided screw terminals to which the ends of 


ELECTRICAL SAFETY DEVICES 


3 


the fuse wire could be connected. Later a copper- 
tipped link was used instead of a simple piece of fuse 
wire, which provided for better connection at the cut¬ 
out terminals. 

When such a fuse is melted by the heating action 
of the current, globules of molten lead are scattered 
about. If the fuse has been melted by a large, sudden 
increase in the current, some of the metal may be 
vaporized, causing an explosive action which may 
throw this molten metal a considerable distance. If 
this occurs near unprotected woodwork or other com¬ 
bustible material, fire may result. It is quite probable 
that in the earlier stages of the electrical industry 
fires have frequently started from this very cause. 

The next step in the course of development was to 
enclose the fuse in some kind of a covering which 
would prevent the scattering of molten metal when 
the wire fused. A great amount of work has been 
done along this line, and many patents have been 
issued in this country and abroad, covering a variety 
of designs. One of the earliest patents on an enclosed 
fuse was issued to Edison in 1880. Fig. 1 gives a 
sketch of this fuse, from which it will be seen to con¬ 
sist merely of a piece of lead wire introduced into a 
gap in the circuit and surrounded by a protecting 
sheath. The wire inside the sheath was surrounded 
with air, which failed to prevent the explosive action 
when the fuse melted. It was, of course, an improve¬ 
ment on the bare wire, but fell very far short of a 
satisfactory enclosed fuse. 

To further reduce the effects of explosive action, 
it was proposed to fill the protecting sheath or tube 
with a chemically inert powder. This was found to 
meet the conditions satisfactorily, and practically all 
modern enclosed fuses are now made in this way. The 
efficiency of the fuse depends to a very large extent 
on the qualities of this filler. In addition to being 


ELECTRICAL SAFETY DEVICES 








ELECTRICAL SAFETY DEVICES 


5 


chemically inert it must be unaffected by atmospheric 
moisture. If the filler tends to set under the action of 
moisture, it virtually forms a hard casing around the 
fuse wire which keeps it in position even after the 
fusing temperature has been reached. Also, the filler 
must be very porous, to provide for the escape of the 
gases generated by the operation of the fuse. Fuse 
manufacturers have now succeeded in producing a 
powder filling which meets these requirements. 

Fig. 2 shows a section of an enclosed fuse which is 
used on circuits of relatively low current-carrying 
capacity. In the figure, A is a small air drum sur¬ 
rounding the middle portion of the fuse wire, which 



has its section somewhat reduced at this point to insure 
that the fuse will melt first inside the drum. The 
object of this form of construction is to produce a 
fuse in which the rise in temperature shall be propor¬ 
tional to the current flowing through it. The fact that 
the air within the drum is a very poor conductor of 
heat causes the fuse to heat rapidly, rendering the 
blowing point practically constant for any given over¬ 
load. On the other hand, if the air drum is not used 
and the fuse wire is surrounded by the filler through¬ 
out its entire length, the rapid conduction of heat 
through the powder causes the blowing time for a 
given overload to vary within wide limits. This results 
from the fact that the ability of the porous powder 
to dissipate heat varies almost as its temperature. 












6 


ELECTRICAL SAFETY DEVICES 


Thus the blowing time for a given overload depends 
upon the rate of heat conduction through the filler, 
and not upon the duration of the overload. 

C in the figure is a small fuse wire which is shunted 
across the main fuse. B is a small hole drilled through 
the protecting tube. The resistance of the shunt wire 
C is large as compared with that of the main fuse, 
hence under normal conditions of operation it carries 
very little 'current. However, when the main fuse 
blows, the shunt receives all the current, causing it to 
open up immediately. Through the small hole B it is 
possible to see when the shunt has been burned out. 
This device is known as the indicator, which enables 
an observer to tell at a glance whether or not any par¬ 
ticular fuse is good or is “ blown.” 

As the voltage of the system and the current capac¬ 
ity of the fuse increase, the fuse element can no longer 
be made of a cylindrical wire, but must be made in the 
form of a flat link, as shown in Fig. 3. 

This link is placed inside the tube in a manner 
similar to that shown in Fig. 2. It has much greater 
surface for a given section than a wire, and hence will 
dissipate heat more rapidly. This is an important 
point under conditions of extreme short circuit where 



Fig. 3. 


a large amount of heat is generated very suddenly. 
In still larger fuses a multiple-link arrangement is 
used, in which two or more links are connected in 
parallel. On account of the large radiating surface 
which the multiple-link arrangement presents, the 
amount of metal used can be reduced. Thus the heat 
developed passes off rapidly, and since less metal 
vapor is formed it can be dissipated more readily with¬ 
out exploding the tube. 










ELECTRICAL SAFETY DEVICES 


7 


In all well-made enclosed fuses the tube is made 
of vulcanized fiber, never of paper. A paper tube 
swells and disintegrates under the action of moisture 
and renders the fuse practically useless. 

Since the introduction of the enclosed fuse in 1890 
a number of companies have engaged in its manufac¬ 
ture. These manufacturers also turned out a line of 
cut-outs to accommodate their fuses, but unfortunately 
there was no agreement among them as to standards 
covering dimensions and details of design. The result 
was that a fuse made by one manufacturer would not 
fit the cut-out made by another; in short, there was 
no interchangeability among the products of the vari¬ 
ous makers. This lack of standards caused great 
inconvenience and confusion, so much so that the 
National Board of Fire Underwriters took up the mat¬ 
ter with the various manufacturers. Standard designs 
and dimensions were then agreed upon for all enclosed 
fuses and cut-outs, so that complete interchangeabil¬ 
ity resulted. Complete fuse equipment can now be 
obtained which conforms to these National Electric 
Code specifications, as they are called. 

Two standard contacts are provided for in these 
N. E. C. specifications, the ferrule clip for capacities 
up to 60 amperes, and the knife-blade contact for 
capacities from 60 amperes to 600 amperes, both for 
250 and 600 volts. Fig. 4 shows a 600-volt, 30-ampere 
fuse with ferrule-clip, contact, and Fig. 5 shows a 
600-volt, 75-ampere fuse with knife-blade contact. The 
figures also show how each fuse is placed in its cut¬ 
out block. 

The National Electric Code specifies the following 
as to the rating of enclosed fuses: “ Fuses must be so 
constructed that with the surrounding atmosphere at 
a temperature of 75° F. they will carry indefinitely a 
current 10 per cent greater than that at which they 
are rated, and at a current 15 per cent greater than 


8 


ELECTRICAL SAFETY DEVICES 


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Fig. 5 



































































































ELECTRICAL SAFETY DEVICES 


9 


the rating they will open the circuit without reaching 
a temperature that will injure the fuse tube or ter¬ 
minals of the fuse block. With a current 50 per cent 
greater than the rating and a room temperature of 
75° F., the fuses, starting cold, must blow within the 
time specified: 

“0- 30 amperes. 1 minute. 

31- 60 amperes. 2 minutes. 

61-100 amperes. . .. 4 minutes. 

101-200 amperes. 6 minutes. 

201-400 amperes.12 minutes. 

401-600 amperes.15 minutes.” 

To make a fuse that will satisfy these conditions 
as to rating is one of the difficult points of manufac¬ 
ture. However, the manufacturers are now producing 
fuses which meet the requirements of the National 
Board of Fire Underwriters in this particular. 

From time to time attempts have been made to 
produce a refillable enclosed fuse; that is, one in which 
the fuse element may be replaced after it has blown 
and the tube or cartridge refilled with the filling pow¬ 
der. There are many difficulties to be overcome in 
the design of a dependable refillable fuse, although in 
time they may be overcome. In the past the attitude 
of the Fire Underwriters toward the refillable fuse 
has generally been one of disfavor. 

One form of fuse not yet mentioned here is the 
well-known Edison fuse plug. It is used with currents 
up to 30 amperes on circuits where the potential does 
not exceed 125 volts. It has a wide application in 
electric-lighting circuits, where the ease with which 
it may be inserted and its reliability make it especially 
valuable. 

The importance of the fuse as an electrical safety 
device can not easily be overestimated. When prop¬ 
erly made and in good condition, it is a reliable safe- 








10 


ELECTRICAL SAFETY DEVICES 


guard against damage due to excessive current. For 
this reason it should be regarded as one of the best 
friends of the electrician and the engineer, and not as 
a necesary evil. When a fuse blows out, replace it 
with another of the same capacity. False ideas of 
economy should never be allowed to dictate the use 
of a fuse of greater capacity than the conditions 
require. It is needless to say that copper wire or a 
nail should never be used, and yet one sometimes sees 
the cut-out block rigged up in this way, in the absence 
of a fuse. Such a practice is bad engineering and may 
prove to be worse economy. 

Questions and Answers. 

Q. What is the chief source of danger to electrical 
equipment? 

A. Greater current than the apparatus is designed 
to stand. 

O. What two effects of the current are made use 
of in protective devices?. 

A. The thermal or heating effect, and the mag¬ 
netic effect. 

Q. What protective device depends upon the ther¬ 
mal effect? 

A. The fuse. 

Q. What device depends upon the magnetic effect? 

A. The circuit-breaker. 

Q. Of what did the first fuse consist? 

A. A small piece of copper wire. 

Q. Why was this unsatisfactory? 

A. Because of the high melting point of copper. 

Q. What material was then used? 

A. Lead and its alloys. 

Q. What objection was there to this fuse? 

A. The danger from molten lead when the fuse 
blew. 


ELECTRICAL SAFETY DEVICES 


11 


Q. Who patented one of the first enclosed fuses? 

A. Edison, in 1880. 

Q. Of what did the fuse consist? 

A. Merely a lead wire enclosed in a sheath or 
tube. 

Q. What was the next step in the evolution of the 
enclosed fuse? 

A. Filling the tube with an inert powder. 

Q. What is an indicator in an enclosed fuse? 

A. A device to show when the fuse is blown. 

Q. Of what does this device consist? 

A. A small wire shunted across the main fuse 
and brought near to a'small hole in the tube. When 
the shunt wire is burned out that fact can be noted 
through the hole. 

Q. Of what material are the tubes made in the 
best fuses? 

A. Vulcanized fiber. 

Q. What kinds of contacts are used in N. E. C. 
standard fuses? 

A. Ferrule clip and knife blade. 

Q. Are fuses and cut-outs from the different manu¬ 
facturers made according to standard specifications? 

A. Yes; N. E. C. specifications. 


12 


ELECTRICAL SAFETY DEVICES 


CHAPTER IT 

One of the most important and widely used safety 
devices in the electrical field is the circuit breaker. 
It is made in a variety of forms to meet the different 
abnormal conditions which may arise in an electric 
circuit, any one of which may be destructive to the 
equipment served by the circuit. These abnormal 
conditions include short circuit, grounding, overload, 
underload, low voltage, current reversal, and phase 
reversal in alternating current. Perhaps the most 
common condition which the circuit breaker is used to 
guard against is an excessive flow of current, caused 
by short circuit, grounding, or overload. Such an 
excessive flow of current will damage the equipment 
connected to the line, and it is the function of the 
breaker to open the circuit before the excessive cur¬ 
rent has time to do damage. 

Unlike the electric fuse, which depends upon the 
heating effect of the current for its action, the circuit 
breaker is operated by the magnetic action of the cur¬ 
rent. Practically all breakers are provided with a 
heavy series coil, through which the total current of 
the line passes. The core of this coil is magnetized 
by the action of the current, the magnetization in¬ 
creasing as the current increases. A soft-iron arma¬ 
ture is attracted toward this magnet, and thus the pull 
on the armature becomes proportional to the current 
flowing in the magnetizing coil. Whenever the cur¬ 
rent exceeds a certain safe limit the armature is drawn 
to the magnet, and this motion releases a trigger which 
holds the breaker in the closed position against the 


ELECTRICAL SAFETY DEVICES 


13 


action of a strong spring. As soon as the trigger 
releases the spring, the breaker is opened suddenly, 
thus breaking the circuit. 

It will be evident that as long as all other condi¬ 
tions remain constant, the amount of current required 
to operate the breaker will vary with the distance 
between the pole of the magnet mentioned above and 
its soft-iron armature. As the distance between mag¬ 
net and armature increases, the magnetic pull on the 
armature becomes less for the same current in the 
coil. Hence to provide enough pull to move the arma¬ 
ture, the current in the coil must be increased. The 
armature may be moved by means of an adjustable 
stop, and hence it is possible to set the breaker to 
operate at any current within its capacity. To set it 
to operate at high current, move the armature away 
from the magnet, and to operate at low current set 
the armature closer to the magnet. 

In considering the structural details of some of the 
most widely used circuit breakers, let us study first 
of all the simple overload single-pole breaker. This 
device is designed to open the circuit Avhenever an 
abnormal flow of current, due to overload or any other 
cause, occurs. It will operate instantly whenever the 
current becomes excessive, without regard to the dura¬ 
tion of the overload. Such a breaker is shown in 
Fig. 6, from which a good idea may be obtained of 
its construction and operation. 

The movable armature is shown at 127, and the 
pin upon which it is pivoted at 128. The magnet 59, 
energized by the winding 50A, exerts a pull on the 
armature 127. When the breaker is closed the arma¬ 
ture is held by gravity away from the magnet 59 and 
against a stop 136, which may be adjusted by the knob 
11. When the current exceeds the limit for which 
the breaker is set, the armature 127 is drawn against 
the magnet 59, revolving on its pivot pin 128, When 


14 


ELECTRICAL SAFETY DEVICES 


making this motion the armature .strikes the latch or 
trigger 87, which in the normal position holds the 
breaker closed against the action of the springs. When 



Fig. 6. 


the latch 87 is thrown out of engagement by the action 
of the armature 127, the breaker is instantly opened 
by the action of the springs. 

When a circuit carrying a heavy current is broken, 
an intensely hot arc is formed which is very destruc¬ 
tive to metal contacts or terminals. It therefore be- 



ELECTRICAL SAFETY DEVICES 


15 


comes necessary to protect the main contact points 
of the circuit breaker from the effects of the arc. This 
is done by using auxiliary contacts of carbon where 
the final break is made. A further consideration of 
Fig. 6 will show how this is done. 

The instrument has two fixed terminals, 98 at the 
top and 50 at the bottom. These terminals are con¬ 
nected to studs which come through the switchboard 
from the rear, to which the main leads from the gen¬ 
erator are attached. The current enters the instru¬ 
ment at the lower contact 50, then passes through 
the overload coil 50A, and into the contact block 50B. 
Then it passes through the laminated copper bridge 
16, thence to the upper contact block 98, through its 
connecting stud to the back of the switchboard, and 
out again on the line. A shunt circuit or by-pass of 
higher resistance than the main bridge 16 is formed 
by the copper strip 3. The current passes from the 
lower contact block through the copper strip 3 to the 
spring plate 30. From there it passes to the secondary 
metallic contacts 69 and 81, and finally to the carbon 
contacts 27 and 75.. 

When the circuit breaker operates the action is 
about as follows: The main bridge 16 first breaks 
contact with the upper block 98. This shunts the 
current through the secondary path formed by the 
strip 3 and the plate 30. This shunt circuit is a suffi¬ 
ciently good conductor to take the whole current and 
prevent the formation of an arc at the main contact 
between 16 and 98. Next the secondary contact be¬ 
tween 69 and 81 is broken and the current passes to 
the carbon contacts 27 and 75, which up to this time 
have been closed. The final break occurs between the 
carbon contacts, which are able to stand the high tem¬ 
perature of the arc without much damage. In this 
way no arc is formed between metallic contacts and 
the life of the contact members is greatly increased. 


16 


ELECTRICAL SAFETY DEVICES 


Practically all electrical equipment is designed to 
take an overload of as much as 50 per cent, and some¬ 
times more, provided it is not continued for any great 
length of time. Such a temporary overload occurs 
on motors which start under load, on feeder lines, 
where the load is variable, and may for a short time 
exceed the rated capacity of the line, and on gen¬ 
erators supplying a number of lighting feeders where 
the load may be suddenly increased, as, for example, 
in the case of a thunderstorm, which causes the con¬ 
sumers to turn on their lamps. Inasmuch as the 
equipment is designed to take care of these temporary 
overloads without injury, it follows that the circuit 
breaker which protects the equipment must be de¬ 
signed to handle the overload without opening the 
circuit. It will be evident, then, that the simple over¬ 
load breaker described above will not be able to meet 
this condition because it is designed to open the cir¬ 
cuit instantly whenever the load exceeds the normal. 

Consider, for example, the case of a generator which 
is designed to carry an overload of 50 per cent for one 
hour without injury. It is not at all likely that the 
machine would be able to carry the same overload 
safely if continued for five hours. Therefore the cir¬ 
cuit breaker which protects this generator must han¬ 
dle an overload of 50 per cent for one hour, but must 
open the circuit if the overload is continued much 
longer than that. 

The requirements which a circuit breaker must 
meet in this kind of service may be stated about as 
follows: On short circuit or very excessive overload 
it must open the circuit instantly. On overloads 
within the capacity of the equipment to handle, it must 
not operate unless the overload be of long duration. 
When the overload is continued for a length of time 
sufficient to endanger the safety of the equipment, 
the breaker must open the circuit. In short, the in- 


ELECTRICAL SAFETY DEVICES 


17 



strument must be affected not only by the magnitude 
of the overload, but also by its duration. 

Several different circuit breakers have been devised 
which embody this time-element feature. One of the 
best of these designs which has come to the notice of 
the writer is shown in Fig. 7. The main features of 


Fig. 7. 


this breaker are practically the same as the simple 
overload breaker shown in Fig. 6, with the addition 
of the time-element feature. Below the housing of 


18 


ELECTRICAL SAFETY DEVICES 


the instrument is placed a small cylindrical vessel 154. 
This vessel contains a specially prepared seat 154a, 
on which rests the disk 184. This disk has a stem 
157, which is attached to the armature 179 of the trip¬ 
ping mechanism. Thus the little disk 184 is attached 
to the magnet which trips the breaker, and as the 
magnet moves it must lift the disk from its seat. 

The manner in which the cylindrical vessel con¬ 
taining the disk is supported is of importance. It is 
held in position by a surrounding ring 182, which has 
two lugs 182a, one on each side. These lugs are con¬ 
nected to the arms 187 of a frame which is pivoted 
on the pin 181 which supports the armature. The two 
arms, each marked 187, carry a plate 156, which co¬ 
operates with a projection of the knob 11. By moving 
the knob 11 to the right or to the left along the cali¬ 
bration plate 13, the vessel 154 is raised or lowered, 
and at the same time the armature 179 is moved 
toward or away from its magnet. Thus the overload 
setting of the breaker can be changed by moving the 
knob 11 on the calibration plate 13. 

The vessel 154 contains a small quantity of special 
oil, which surrounds the disk and its seat, thus exclud¬ 
ing all air from between the contact surfaces,'which 
are separated only by a very thin film of oil. The 
surface tension of this oil film causes the disk to 
adhere to its seat, so that a considerable pull is re¬ 
quired to separate them. 

The operation of this time-element feature is as 
follows: When an overload occurs, the pull of the 
armature tends to lift the disk from its seat, thus sub¬ 
jecting the oil film to tension. If the overload be con¬ 
tinued for a considerable time the pull will finally 
rupture the oil film, the disk will be lifted from its 
seat, the armature is attracted to its magnet and the 
breaker is tripped, opening the circuit. In the case 
of a short circuit or very heavy overload, the pull of 


ELECTRICAL SAFETY DEVICES 


19 


the armature is sufficient to rupture the oil film at 
once, and the breaker operates instantly. 

In order to pull the disk from its seat it must be 
moved a certain small distance against the surface 
tension of the oil film; that is, a certain definite amount 
of work must be done. A comparatively small over¬ 
load, long continued, pulls steadily on the disk, until 
finally the oil film breaks. If the overload is relieved 
before rupture occurs, the pull on the disk becomes 
less, and it falls back on its seat again. Thus a 
breaker like this meets the conditions of operation out¬ 
lined above. It is known as the inverse time-element 
circuit breaker, because the time required to trip it is 
inversely proportional to the magnitude of the over¬ 
load. 

When two or more generators are operating in par¬ 
allel a condition may arise which neither the overload 
circuit breaker nor the time-element instrument is able 
to meet. In case of accident to one generator or to 
its prime mover it may not only fail to deliver its por¬ 
tion of the power but may even draw current from 
the line and thus operate as a motor. Hence a reversal 
of current in the generator circuit must be guarded 
against. The same condition may arise in connection 
with rotary converters, storage batteries and charging 
sets. 

In order to meet this condition a reverse-current 
circuit breaker has been designed, which operates 
when the current in the circuit reverses, and thus 
throws the machine out of circuit. Fig. 8 shows a 
breaker of the ordinary overload type, provided with 
the time-element feature and also having the reverse- 
current attachment. Such a breaker will protect 
against sudden heavy overload, continued overload 
and reverse current. The reverse-current relays are 
shown at the bottom of the instrument. Fig. 8. 

The two coils shown in the figure are connected as 


20 


ELECTRICAL SAFETY DEVICES 



Fig. 8. 


t 

0 


\ 


































ELECTRICAL SAEETY DEVICES 


21 


shunts to the main circuit and are wound in opposite 
directions, giving them opposite polarity. Between 
the poles of these two shunt magnet coils is placed the 
pole of a magnet, energized by a series-wound coil. 
The core of the series coil is movable and may be 
attracted toward either core of the shunt coils. When 
the current flows in the proper direction the series 
coil and the lower shunt coil have opposite polarity 
and their cores are therefore attracted toward each 
other. When a reversal of current takes place the 
polarity of the series coil is reversed, while that of the 
shunt coils is unchanged. The core of the series coil 
is now attracted toward the upper shunt coil, and this 
motion trips the breaker. Thus the circuit is broken 
at the instant the current reverses. 

Questions and Answers. 

Q. Upon what effect of the electric current does 
a circuit breaker depend for its action? 

A. Upon the magnetic effect. 

Q. How is this magnetic effect made use of? 

A. The circuit breaker is provided with a series 
coil; a soft-iron armature is attracted toward the core 
of this coil whenever the current becomes excessive. 
The motion of this armature trips a trigger and a 
spring opens the breaker. 

Q. What is the simplest kind of circuit breaker? 

A. One which provides protection against excess¬ 
ive current, due to short circuit or heavy overload. 

Q. Is such a breaker always satisfactory? 

A. No; it is often desirable to carry a small over¬ 
load for a time and the overload breaker would open 
the circuit. 

Q. What kind of a breaker is used in such a case? 

A. A time-limit breaker, which opens the circuit 
in a time inversely proportional to the magnitude of 
the overload. 


22 


ELECTRICAL SAFETY DEVICES 


Q. Will such a breaker operate instantly on short 
circuit or dangerous overload? 

A. Yes. 

Q. What usually occurs when a circuit carrying 
a heavy current is broken? 

A. An arc is formed. 

Q. How is a circuit breaker made to prevent dam¬ 
age to the metallic contacts by the action of the arc? 

A. Auxiliary carbon contacts are used, and the 
final break is made there. 

Q. What is a reverse-current breaker? 

A. One which opens the circuit when the direc¬ 
tion of current flow is reversed. 

Q. Where is such a breaker used? 

A. On generators running in parallel, on storage 
batteries, and on battery-charging sets. 

Cuts supplied through courtesy of the Cutter Company. 


ELECTRICAL SAFETY DEVICES 


23 


CHAPTER III. 

Lightning Arresters. 

An important line of electrical safety equipment 
has been developed for the protection of overhead 
transmission lines, designated by the general term 
“ lightning arresters.” Used in this connection, the 
word “ lightning ” means much more than is implied 
in the popular definition of the term. It may be taken 
to mean any abnormal condition of voltage or fre¬ 
quency, produced by external or internal causes, tend¬ 
ing to interfere with the proper operation of the 
system. 

Some of these conditions, which are all grouped 
under the one term “ lightning,” are as follows: 

1. An electric discharge between a cloud and the 
earth which strikes the transmission line. This is 
lightning in the ordinary meaning of the term. 

2. The induction, on the transmission line, of an 
electrostatic charge of high potential from a heavily 
charged cloud. This charge may or may not be suffi¬ 
cient to produce a discharge between the cloud and 
the line. 

3. The gradual accumulation of a charge upon the 
line from rain, snow or mist. This occurs particularly 
during a thunderstorm when the raindrops all carry 
a static charge. 

4. Pressure disturbances due to sudden changes 
in the load, such as throwing machines on or off. 

5. Surges and oscillations on the line, the initial 


24 


ELECTRICAL SAFETY DEVICES 


cause of which can probably be traced to short-cir¬ 
cuits, grounds, discharges at faulty insulators, etc. 

All of the phenomena enumerated above produce 
high potential charges upon the line, which in time 
may become sufficient to cause a discharge to the 
earth or any other adjacent object. As soon as the 
discharge takes place with the formation of an arc, 
the dielectric resistance of the air is greatly reduced, 
which makes it possible for the arc to be maintained 
at a potential difference much less than that at which 
it was started. The result is that a virtual “ ground ” 
is produced by the arc unless something is done to 
interrupt it. 

To meet this condition one of the first lightning 
arresters that ever came into practical use was in¬ 
vented by Prof. Elihu Thomson in 1884. The idea 
of this arrester can be seen from Fig. 9, which shows 
an outline of the essential parts. The overhead trans¬ 
mission line is shown at L; aa is a single gap across 
which the charge must arc in order to find its way to 
the ground G. M is an electromagnet energized by 
the coil C, through which any current following a 
static discharge between the terminals aa must pass 
on its way to the ground. The result is that the arc 
is immediately blown out by magnet M, in the field 
of which the arc was originally formed. Thus this 
device allows a heavy static charge to escape to the 
earth, but will not permit much current to escape 
because the magnet blows the arc out. 

It may be of interest at this point to consider just 
what is the action of this magnet in blowing out the 
arc. In the first place it must be remembered that the 
arc can only be maintained by the flow of current 
across the gap between the two terminals. That is, 
the gap is really a conductor carrying a current, 
although, of course, the resistance is very high. We 
have then in the case of the blow-out magnet, a con- 


ELECTRICAL SAFETY DEVICES 


25 


ductor carrying a current at right angles to a mag¬ 
netic field. Now the fundamental principle of the 
electric motor is that a conductor carrying a current 


L 



a 



G 


Fig. 9. 


and so placed with respect to a magnetic field is 
moved across the lines of force. When this particular 
conductor is moved across the lines of force it is bent 
more and more, since the terminals aa can not move, 
until finally it is broken and the arc extinguished. 

This type of lightning arrester is now used con¬ 
siderably in direct-current work, where the pressure 
does not exceed 6,000 volts. The length of the spark 

















26 


ELECTRICAL SAFETY DEVICES 


gap is usually made adjustable so that it can be used 
for different voltages. 

The magnet blow-out also has considerable appli¬ 
cation in electric railway work, where it is used to 
blow out the arc formed when contacts are broken 
in the controller. Every time the motorman starts 
the car, sliding contacts are made and broken and the 
contact strips and fingers would be greatly damaged 
by the arc if provision were not made to blow it out. 
The coil of the blow-out magnet is connected in series 
so that the strength of the magnetic field varies with 
the strength of the current which must be interrupted. 

When we come to transmission lines of high poten¬ 
tial the single-gap arrester with magnetic blow-out 
is no longer satisfactory. To meet this condition the 
multigap arrester is used perhaps more widely than 
any other. It consists of a number of small brass 
cylinders, mounted in a row on an insulated base with 
small gaps between them. The cylinder on one end of 
the row is connected to the transmission line, and the 
one at the other end connected to the ground. The 
action of this arrester is about as follows: 

These little cylinders are all charged inductively 
from the line, the intensity of the charge on each 
being proportional to its distance from the line. That 
is, the one nearest the line will have a maximum 
charge and the one nearest the ground will have prac¬ 
tically a zero charge. When the potential difference 
between the first and second cylinders becomes suffi¬ 
cient to break down the resistance of the intervening 
air a discharge takes place between them. The second 
cylinder is now connected to the first by means of 
the arc, and its potential may be raised sufficiently 
to produce a discharge between it and the third. In 
the same way the discharge may continue from the 
third cylinder to the fourth, and so on, until all the 
gaps have been bridged. The charge then escapes 


ELECTRICAL SAFETY DEVICES 


27 


to the ground and current begins to flow through the 
successive arcs from the line to the ground. There 
is now a uniform drop in potential across the arrester 
from a maximum at the line to zero at the ground. 
The potential necessary to maintain the arc is less 
than was required to start it because of the weaken¬ 
ing of the dielectric resistance of the air due to the 
heat of the arc. 

The arc across the arrester might therefore con¬ 
tinue for a considerable time, except for one impor¬ 
tant fact. An alternating e. m. f. passes through the 
value zero twice in each complete cycle. After the 
high-potential charge has dissipated itself in the 
arrester or has escaped to the ground there will be 
insufficient potential to form the arc again after it has 
gone out at the end of a half cycle. Thus in this case 
no magnetic blow-out is needed, because the arc puts 
itself out when the e. m. f. becomes zero. 

The behavior of a multigap arrester is somewhat 
aflfected by the frequency. As the frequency increases, 
the potential at which the arrester will discharge de¬ 
creases. In order to make an arrester which will 
work satisfactorily on different frequencies, resistances 
are connected in shunt with the gaps as shown in 
Fig. 10. In the figure, is a low resistance shunted 
across a portion of the gaps; Rg is a somewhat higher 
resistance shunted across still more of the gaps, and 
R^ is a high resistance connected across almost the 
whole of the arrester. The action of a multigap 
arrester with shunt resistances as shown is about as 
follows: 

In the case of high frequency the discharge can pass 
more readily across the gaps than through the shunt 
resistance, hence it will take the direct path across 
the arrester. The fact that self-induction of the resist¬ 
ance increases with the frequency is another reason 
why a high-frequency discharge passes across the 


28 


ELECTRICAL SAFETY DEVICES 


arrester gaps instead of through the shunt resistance. 
When the frequency is lower the discharge passes 
through the low resistance Ri and then across the 
gaps to the ground; if the frequency is still lower 



? 


“G. 

Fig. 10 . 

the discharge passes through the intermediate resist¬ 
ance R, and then across the remaining gaps. Finally, 
in the case of a still lower frequency it passes through 
the high resistance Rg and then across the few remain- 








ELECTRICAL SAFETY DEVICES 


29 


ing gaps. Thus such an arrester provides protection 
against high-potential disturbances of almost any fre¬ 
quency. 

It must not be supposed that the only frequency 
which may occur on a transmission line is the normal 
frequency of the system as determined by the gen¬ 
erators. Lightning disturbances may produce on a 
transmission line very high frequency oscillations, 
many times the normal frequency. On the other hand, 
sudden changes in load, short circuits, grounds, etc., 
may produce a swinging or surging of the power on 
the line which is of relatively low frequency. Hence 
a lightning arrester may have disturbances to guard 
against, the frequencies of which vary through a wide 
range. 

In some cases a resistance is connected in series 
with a multigap arrester, although it is not usually 
considered good practice. A resistance in series will 
cut down the current flowing across the arrester and 
so prove a protection to the latter. But in the case 
of a very high-frequency disturbance the inductive 
action of the resistance would choke back the current 
and so prevent the arrester from operating. In gen¬ 
eral, series resistances are not used. 

It sometimes happens that a high-frequency oscil¬ 
lation will enter a generating station and do damage 
to the equipment even although the system be sup¬ 
plied with lightning arresters. In such cases the dam¬ 
age is done before the lightning arrester has time to 
operate. To meet such a condition as this, reactance 
coils are connected into the line where it enters the 
station. Part of the wave is reflected by the reactance 
coil and then finds its way to the earth through the 
lightning arrester. Part of it will, of course, pass 
through the coil, but it must not be enough to break 
down the insulation of the station equipment. Thus 
if the coil is properly designed it protects against 


30 


ELECTRICAL SAFETY DEVICES 


high-frequency disturbances. On the other hand, it 
affords no protection against low-frequency surges, 
because the reactance of the coil must be limited to 
that allowable for the normal voltage of the line. 

Questions and Answers. 

Q. What is a lightning arrester? 

A. A device to protect a transmission line and the 
equipment connected thereto from lightning. 

Q. What is the meaning of the term lightning as 
used in this connection? 

A. Any abnormal pressure condition, whether 
produced for external or internal causes. 

Q. What are some external causes which may 
produce such a condition? 

A. A direct stroke of lightning striking the line; 
the induction of a heavy charge upon the line from a 
near-by cloud; or the accumulation of a charge from 
rain, snow, etc. 

Q. What are some internal causes which may pro¬ 
duce a similar condition? 

A. Sudden changes in load; a short-circuit; a 
ground; a discharge from a lightning arrester, etc. 

Q. Who invented the first lightning arrester to 
come into extensive practical use? 

A. Prof. Elihu Thomson, in 1884. 

Q. Of what did this arrester consist? 

A. It consisted of a single spark gap with mag¬ 
netic blow-out. 

Q. How was it connected to the line? 

A. One side of the gap was connected to the line, 
the other side to the ground, with the coil of the blow¬ 
out magnet in series with the gap. 

Q. What is the purpose of the blow-out magnet? 

A. To blow out the arc formed by a discharge 
across the gap. 


ELECTRICAL SAFETY DEVICES 


31 


Q. How is this done? 

A. The arc, which is a conductor, is at right 
angles to the field of the magnet. The arc is moved 
across the field, just as any conductor carrying a cur¬ 
rent would be, until it is broken. 

Q. For what kind of work is this arrester used? 

A. For direct currents up to 6,000 volts potential. 

Q. For high-potential alternating-current work 
what kind of arrester is used? 

A. The multigap arrester. 

Q. How is it made? 

A. It consists of a number of small cylinders of 
brass or special alloy, mounted close together but not 
touching, on an insulated base. 

Q. How is it connected to the line? 

A. One end is connected to the line, the other to 
the ground. 

Q. How does such an arrester work? 

A. The discharge forms an arc from one cylinder 
to the next, and so on, until the charge is dissipated 
in the arrester or escapes to the ground. 

Q. Why is no blow-out magnet needed with this 
arrester? 

A. Because an alternating e. m. f. passes through 
zero once in each half cycle. When this occurs the 
arc goes out, and the potential is not sufficient to start 
it again. 

Q. What is the value of a reactance coil as a pro¬ 
tective device on a transmission line? 

A. On account of its reactance the coil throws 
back a high-frequency disturbance, which may then 
escape to the ground through the lightning arrester. 

Q. Where are these coils placed on the line? 

A. Near the station, to protect the equipment. 


32 


ELECTRICAL SAFETY DEVICES 


Q. Will such a coil protect from low-frequency 
disturbances? 

A. No; the reactance is not sufficient; if in¬ 
creased, the reactance would interfere with the normal 
operation of the system. 


ELECTRICAL SAFETY DEVICES 


33 


CHAPTER IV. 

Regulators. 

A regulator is a device for controlling the voltage 
either at the generator or on a feeder near the center 
of distribution. Although, strictly speaking, the volt¬ 
age regulator is not a safety device, still it partakes 
of the nature of such equipment, particularly when it 
is automatically operated. 

In any large electrical distributing system it is 
impossible to have satisfactory voltage regulation at 
all points simply by control of the generator voltage 
at the central station. Some feeders may be long, 
while others are short; the load on one may be fairly 
uniform, while on another it may be subject to wide 
variations. Hence it becomes necessary to control 
the voltage on each individual feeder if the regulation 
is to be satisfactory at all times at all points of the 
system. 

To meet this condition, feeder regulators have been 
devised which may be placed at any desired point on 
each individual feeder. These regulators are simply 
transformers or compensators, in which the ratio of 
transformation is variable by changing the number 
of turns of wire in circuit in the secondary winding. 
These regulating transformers are connected with the 
primary winding across the circuit to be controlled 
and the secondary in series with it. 

Figs. 11 and 12 show diagrammatically the arrange¬ 
ment of the windings and the manner in which they 
are connected into the line. Fig. 11 shows a regulator 


34 


ELECTRICAL SAFETY DEVICES* 


arranged to boost the voltage of the line, and Fig. 12 
shows one which is arranged in such a way as to 
lower the line voltage. Of course, the same regu¬ 
lator is capable of both boosting and lowering effect. 

In Fig. 11 the current in the primary or shunt coil 


^00Vo/t5 /OOAmiD3, nOVo/t5SO.S//irr7^ . 


^huntWind/n^ 
rrom SOS/Imps. 
Generator 


To 

reeder 


WWW- 


Series Winding 


Fig. II. 


is in such a direction that it induces a pressure in the 
series or secondary coil in the same direction as the 
line pressure. This induced pressure is therefore 
added to the line pressure, which gives the regulator 
a boosting effect. In Fig. 12 the current flows in the 


WOVb/t^ /OOAmp^ 


SO Voitsi/iJiAmp^- 


ShuntWfnding 

rrorn 

Generator 


To 

reeder 


WWW— 

Ser/es Wincf/ng 

Fig. 12 . 


opposite direction through the shunt coil. This in¬ 
duces a pressure in the series coil which is opposite 
in direction to the line pressure. Thus the line pres¬ 
sure is reduced, giving the regulator a lowering effect. 








ELECTRICAL SAFETY DEVICES 


35 


The regulator shown in the figures is designed to 
have a 10 per cent boosting or 10 per cent lowering 
effect. That is, the windings are proportioned in such 
a way that the maximum voltage generated in the 
secondary by the inductive action of the primary is 
just 10 per cent of the line pressure. In this particu¬ 
lar case a circuit of 100 volts and 100 amperes is 
assumed. In Fig. 11 the voltage boost is 10 per cent 
of 100 volts = 10 volts, and the line pressure beyond 
the regulator therefore becomes 100 10 = 110 volts. 

Now, before the regulator was reached, that is, to 
the left of the regulator in the figure, the total power 
on the line was 1&) X 100 = 10,000 watts. Disregard¬ 
ing the slight loss which occurs in the regulator, the 
power is the same after the current has passed through 
it. Therefore the current beyond the regulator equals 


10,000 

110 


90.9 amperes. 


In Fig. 12 the secondary winding causes a reduc¬ 
tion of 10 volts in the line pressure, which therefore 
becomes 100 — 10 = 90 volts. In this case the current 

beyond the regulator becomes ——= 11.11 amperes. 


This regulator is, of course, capable of any inter¬ 
mediate effect between 10 per cent boost and 10 per 
cent lower. This effect is obtained by reducing the 
number of turns of wire in circuit in the secondary 
coil, or by changing the position of the primary and 
secondary coils with respect to each other, which 
changes their mutual inductive effect. The well- 
known induction regulator produces its effect that way. 

The induction regulator, like the one just de¬ 
scribed, has two windings, a primary across the line 
and a secondary in series with the line. Fig. 13 shows 
the arrangement of these windings. The primary or 
shunt coil is wound in slots on the outside circumfer- 



36 


ELECTRICAL SAFETY DEVICES 


ence of a movable iron core, and the secondary or 
series coil is wound in similar slots on the inside of 
a stationary iron core. The voltage regulation is 
obtained by changing the relative position of the pri¬ 
mary and secondary coils by revolving the inner core. 
When a positive pole of the primary coil is opposite 
a positive pole of the secondary, a regular transformer 
action is produced and the voltage generated in the 
secondary boosts the line pressure. When a positive 
pole of the primary is opposite a negative pole of the 
secondary the voltage induced in the secondary is 
opposite in direction to what it was before, and the 
line pressure is therefore lowered. In a position of 
the movable coil midway between these two extremes 
the primary and secondary coils are out of inductive 
relation to each other, no pressure is induced in the 
secondary, and the regulator neither boosts nor lowers. 

Perhaps a clearer idea of this action of the regu¬ 
lator may be obtained from the following considera¬ 
tions. The secondary coil, being connected in series 
with the line and wound on a stationary core, produces 
a flux which is constant in magnitude and direction. 
The direction of this flux is such as at all times to 
oppose the current which produces it. Now, whenever 
the flux from the primary opposes the secondary flux, 
it will induce in the secondary a voltage in the same 
direction as the line voltage, thus boosting the latter. 
This is just what takes place when similar poles on 
the primary and secondary coils are opposite each 
other. When unlike poles are opposite each other 
the effect is the reverse, as already explained. The 
whole effect is produced by these two fluxes, primary 
and secondary, acting against each other, with each 
other, or at some intermediate position between these 
two extremes. 

By referring again to Fig. 13 it will be noted that 
the inside movable core has a short-circuited coil 


ELECTRICAL SAFETY DEVICES 


37 


wound at right angles to the primary shunt coil. The 
object of this coil is to decrease the reactance of the 
regulator, the principle of its operation being as fol¬ 
lows : When the regulator is in the neutral, or no 



Fig. 13. 


boost, no lower position, the primary and secondary 
coils are at right angles to each other, and hence have 
no mutual inductive relation. In this position none 
of the primary flux passes through the secondary, and 

























38 


ELECTRICAL SAFETY DEVICES 


hence considerable potential would be required to 
force the line current through it; that is, through 
the secondary or series winding. This voltage would 
be at right angles to the line voltage, and hence a 
poor power factor would be the result. 

The short-circuited coil, however, is in inductive 
relation with the secondary when the. regulator is in 
the neutral position. The flux from the short-cir¬ 
cuited coil, cutting the secondary winding, acts like 
a short circuit on the latter, and thus greatly reduces 
the voltage necessary to force the load current through 
it. Since the secondary is always in inductive rela¬ 
tion with either the main primary winding or the 
short-circuited winding, it is always subject to more 
or less short-circuiting effect, which keeps the react¬ 
ance down to a point where only a small amount of 
pressure is necessary to force the current through it. 

Fig. 14 shows an external view of a single-phase 
induction regulator, the construction of which has 
just been described. The coils are mounted in an 
iron case very similar to that used for the ordinary 
transformer. Methods of cooling are practically the 
same as for transformers, but in many cases such a 
regulator operates satisfactorily when self-cooled. 

The construction of a three-phase induction regu¬ 
lator is practically the same as the single-phase type, 
although the operation is somewhat different. The 
primary winding consists of three shunt coils, one 
across each phase of the system. The windings are 
arranged symmetrically in the slots of the inner mova¬ 
ble core of the regulator, very much like the windings 
on a three-phase generator armature. For this reason 
the primary of the regulator is often spoken of as the 
armature. The flux produced by this three-phase 
primary winding is not constant in direction as in the 
case of the single-phase coil, but is revolving like the 
flux in the stator of an induction motor. 


ELECTRICAL SAFETY DEVICES 


39 


The series or secondary winding is arranged in 
slots on the inside of the stationary core, in the same 
manner as the primary. This winding consists of 
three separate coils, one for each phase. The volt- 



Fig. 14. 


ages induced in the secondary are due to the revolving 
flux of the primary, produced by the combined action 
of the three phases. The secondary induced voltages 
are constant at all times and for any position of the 











40 


ELECTRICAL SAFETY DEVICES 


armature or primary, because the primary flux is 
constant in effect. The variations in the line voltage 
produced by the regulator are due to phase displace¬ 
ment between the secondary voltage and the line 
voltage. 

Suppose that primary winding, phase 1, is just 
opposite the corresponding coil on the secondary. 
Then the action will be similar to the single-phase 
regulator and the voltage generated in this particular 
phase of the secondary will be added directly to the 
line voltage, giving a boosting effect. Now, if the 
primary coil be rotated slightly, phase 1 of the sec¬ 
ondary will be acted upon by both phase 1 and phase 
2 of the primary. This will produce a voltage in the 
secondary of the same magnitude as before, but some¬ 
what out of phase with the line voltage. In this case 
the boosting will be less than before. Thus by rotat¬ 
ing the primary any effect from maximum boost to 
maximum lower may be obtained. 

Fig. 15 is a view of a three-phase induction regu¬ 
lator. The small motor on the top of the case is for 
revolving the regulator armature or primary. The 
control switch for the motor may be mounted on the 
switchboard, and the operator may boost or lower 
the feeder pressure simply by throwing the control 
switch up or down. A wheel for hand control is also 
provided, if for any reason the motor can not be used. 

A regulator of this kind may also be automatically 
controlled, in which form it partakes of the nature of 
a safety device. This automatic control is accom¬ 
plished by means of a contact-making voltmeter, 
shown in Fig. 16. This contact-making instrument 
consists essentially of a solenoid with a laminated 
iron core, both shown clearly in the figure. The core 
is supported by a spring and also by means of the 
current in the solenoid. The core of the solenoid 
operates a lever having contact points above and below. 


ELECTRICAL SAFETY DEVICES 


41 














































42 


ELECTRICAL SAEETY DEVICES 


With the voltage at the proper point the lever 
stands midway between the two contacts. A change 
in voltage causes a movement of the core of the sole¬ 
noid, moves the lever up or down, and closes one or 
the other of the contacts. This operates a relay 



Fig. i6 . 

switch, which closes the circuit on the control motor 
of the regulator. When sufficient regulation has been 
obtained, the contact-making lever drops back again 
into the neutral position, the contact is broken and 
the motor stops. Thus the voltage regulation becomes 
entirely automatic. 









ELECTRICAL SAFETY DEVICES 


43 


Questions and Answers. 

Q. What is a regulator? 

A.^ A device for controlling the voltage of an elec¬ 
tric circuit. 

Q. Of what does an induction regulator consist? 

A. It consists of two coils, a primary and a sec¬ 
ondary, very much like a transformer. 

Q. How are these windings connected? 

A. The primary is shunted across the line, and 
the secondary in series with the line. 

Q. How are these coils placed with respect to 
each other? 

A. They are wound on concentric laminated iron 
cores, the primary on the outside of the inner movable 
core, and the secondary on the inside of the outer sta¬ 
tionary core. 

Q. How does such a device control the voltage 
of the line? 

A. By means of the pressure induced in the sec¬ 
ondary by the flux from the primary. This pressure 
is variable in amount and direction, depending upon 
the relative position of primary and secondary cores. 

Q. How is a variable regulation accomplished 
with such a device? 

A. Simply by turning the movable primary core 
inside the stationary secondary. 

Q. How is this turning done? 

A. By hand or with a motor. 

Q. Can a regulator of this kind be automatically . 
controlled? 

A. Yes; by means of a contact-making voltmeter 
and relay switch. 


44 


ELECTRICAL SAFETY DEVICES 


CHAPTER V. 

In Chapter IV it was pointed out that any auto¬ 
matic voltage regulator partakes of the nature of a 
safety device, since it guards against voltage fluctua¬ 
tions that might easily prove serious if allowed to go 
unchecked. There the different forms of induction 
regulators were discussed, and it was made plain that 
such an instrument is essentially a feeder regulator, 
designed to regulate the voltage on each individual 
outgoing line. 

It is also very important, however, to have the gen¬ 
erator in the power plant controlled by an automatic 
voltage regulator, so that the pressure will be main¬ 
tained constant throughout all load fluctuations within 
the capacity of the machine. The instrument which 
probably has the widest application in this field is 
known as the Tirrill regulator, named after its in¬ 
ventor. This device is made for use with either direct 
or alternating current machines; in the case of the 
d. c. machine the regulator works by automatically 
short-circuiting the generator field rheostat, while in 
the case of the a. c. generator it short-circuits the 
exciter field rheostat. 

Fig. 17 gives a diagram of the connections for a Tir¬ 
rill regulator, designed for use with a d. c. machine. 
The essential parts of the instrument are the main 
control magnet and main contacts, shown in the upper 
left-hand corner of the drawing, and the differentially 
wound relay and relay contacts, shown at the right- 
hand side of the figure. The main control magnet is 
simply a solenoid provided with an iron core, and con- 


ELECTRICAL SAFETY DEVICES 


45 



/3(/3 Sara 

















































46 


ELECTRICAL SAFETY DEVICES 


nected permanently across the bus bars as shown. A 
coiled spring, shown above the main control magnet, 
opposes the action of the latter, and the two working 
against each other control the opening and closing of 
the main contact. 

The two coils on the differentially wound relay are 
wound in opposite directions, so that the magnetism 
of one opposes that of the other. The left-hand coil 
of this relay will be seen from the figure to be con¬ 
nected permanently across the bus bars, while the 
right-hand coil is only in circuit when the main con¬ 
tact is closed. The action of the relay magnet is 
opposed by a coiled spring in practically the same man¬ 
ner as in the main control magnet. A condenser is 
connected across the relay contacts to reduce the 
sparking which might otherwise prove destructive at 
the opening of the contacts. 

The drawing also shows the generator connected 
to the bus bars, the generator shunt field, and the field 
rheostat. It should be remembered that the regulator 
accomplishes its function by short-circuiting this field 
rheostat. 

The action of the regulator is about as follows: 
Suppose that the generator voltage is about normal 
and that the main contact is open. Only the left-hand 
coil of the differential relay is receiving current, be¬ 
cause the right-hand coil is not cut into circuit until 
the main contact is closed. Under this condition the 
differential relay holds the relay contact open against 
the action of the coiled spring. It will also be noted 
that generator field rheostat is in circuit with its 
machine. 

Now, suppose that the generator voltage drops, 
due to increase in load. The coil of the main control 
magnet will now receive less current than before, 
because the pressure is less. Its pull will, therefore, 
become less and will be overcome by the spring above, 


ELECTRICAL SAFETY DEVICES 


47 


thus closing the main contact. As soon as this con¬ 
tact is closed, the right-hand coil of the differential 
relay is cut into the circuit. Since it is wound in the 
opposite direction from the left-hand coil, the mag¬ 
netism of the former destroys that of the latter, and 
the pull of the relay becomes zero. The coiled spring 
above immediately pulls up and closes the relay con¬ 
tact. It will be seen from a study of the figure that 
as soon as the relay contact closes the field rheostat 
is short-circuited and thus thrown out of circuit. This 
immediately boosts the generator voltage, the main 
control magnet receives more current, which enables 
it to open the main contact. This throws the right- 
hand coil of the differential relay out of circuit, the 
relay contact is opened, which breaks the short circuit 
on the field rheostat, putting it into circuit again. 

During the operation oi such a regulator the dura¬ 
tion of the short circuit of the field rheostat is very 
short. In fact, both main and relay contacts are con¬ 
stantly opening and closing. 

Sometimes the main control magnet is supplied 
with a series coil in addition to the shunt coil. This 
series coil is connected in such a manner as to oppose 
the magnetism of the shunt coil. Thus, when the load 
becomes very heavy, the main control magnet be¬ 
comes weak, due to the opposing action of the series 
coil. This enables the spring to keep the main contact 
closed for a longer time than would otherwise be pos¬ 
sible, thus raising the voltage as the load increases. 
The effect is similar to that obtained from an over¬ 
compounded machine. 

Of course, the series coil mentioned here does not 
receive the whole of the main current, but only a por¬ 
tion of it. A low-resistance shunt is connected in 
series with the line, and the series coil is then con¬ 
nected across this shunt, similar to the method used 
in connecting direct-current ammeters. 


48 


ELECTR[CAL SAFETY DEVICES 



od 


e5 
















































































ELECTRICAL SAFETY DEVICES 


49 


Fig. 18 shows a diagram of the Tirrill regulator, as 
designed for use with an alternating-current generator. 
It will be noted that in this case the field rheostat of 
the exciter is short-circuited instead of the rheostat 



Fig. 19. 


in the field of the machine itself. The regulator in 
this form operates in the same manner as the d. c. type, 
except that it is provided with an additional control 



























50 


ELECTRICAL SAFETY DEVICES 


magnet, which is supplied with alternating current 
from the main bus bars. This alternating-current con¬ 
trol magnet is shown in the upper right-hand corner 
or Fig. 18. 

The operation of this magnet is as follows: It will 
be noted that it is provided with two coils, a series 
winding and a pressure winding. The former is sup¬ 
plied with current from a current transformer, the 
primary of which is connected in series with one of 
the main bus bars. The latter — that is, the pressure 
winding — is supplied by a potential transformer, the 
primary of which is connected across one phase of the 
main alternating-current circuit. Whenever the volt¬ 
age on the a. c. side rises above normal, the a. c. con¬ 
trol magnet receives more current and is drawn up¬ 
ward, due to its increased magnetism. This brings 
the main contacts farther apart, and so reduces the 
time of short circuit of the exciter field rheostat. In 
this way the normal boosting effect of the regulator is 
reduced somewhat. 

The series coil of the a. c. control magnet is con¬ 
nected in such a way that its magnetism opposes that 
of the pressure coil, the result being to weaken the 
total pull of the magnet. When the load on the a. c. 
side becomes heavy, the differential effect of the series 
coil is increased and the a. c. control magnet becomes 
weaker. The effect is to permit of a longer closing 
of the main contact, which results in a longer duration 
of the short circuit on the exciter field rheostat. Thus 
an increased boosting or over-compounding effect is 
obtained by the action of the series coil on the a. c. 
control magnet, in practically the same way as has 
already been described for the d. c. control magnet. 

Fig. 19 gives an exterior view of the Tirrill regu¬ 
lator fully assembled. The d. c. control magnet is at 
the left, the a. c. magnet at the right, with the main 


ELECTRICAL SAFETY DEVICES 


51 


contacts between them. At the bottom is shown the 
differential relay and relay contacts. 

Questions and Answers. 

Q. What is the function of a Tirrill regulator? 

A. To automatically control the generator voltage. 

Q. How is this control accomplished? 

A. By short-circuiting the field rheostat. 

Q. Can such a device be used for either d. c. or 
a. c. machines? 

A. Yes; although there is a slight difference in 
design between the a. c. and d. c. regulator. 



ALPHABETICAL INDEX 


Arc in Circuit-breaker. 15 

Blow-out Magnet . 24 

Action of.24-25 

Circuit-breaker. 12 

Action of ..13, 15 

Principle of. 12 

Reverse-current. 19 

Structural Details of. 13 

Time Element of. 17 

Contact-making Voltmeter. 40 

Operation of. 42 

Cut-outs.. 7 

Specifications for. 7 

Edison Fuse Plug. 9 

Edison’s Enclosed Fuse. 3 

Enclosed Fuse. 5 

Construction of.5-6 

Fuse Elements in.5-6 

Indicator for. 6 

Electric Railway: 

Magnetic Blow-out in Connection with. 26 

Feeder Regulator . 33 

Construction of. 34 

Operation of.34-35 

Filler for Fuses. 3 

Frequencies on Transmission Line. 29 



























54 


ELECTRICAL SAFETY DEVICES 


Fuse . 2 

Early Design .2-3 

Importance of. 9 

Material First Used for. 2 

Importance of Fuse. 9 

Indicator for Enclosed Fuse. 6 

Induction Regulator. 35 

Action of. 36 

Short-circuited Coil on.36-37 

Single-phase. 38 

Three-phase . 38 

Lightning: 

Definition of. 23 

Lightning Arresters. 23 

Inventor of. 24 

Multi-gap . 26 

Single-gap. 25 

Multi-gap Arrester. 26 

Effect of Frequency on. 27 

Operation of.26-27 

Shunt Resistances with.27-28 

Rating of Fuses. 7 

Reactance Coils. 29 

Refillable Fuse. 9 

Reverse-current Breaker. 19 

Construction of. 21 

Operation of. 21 

Regulators . 33 

For Feeders . 33 

Induction Type . 35 

Tirrill. 44 

Single-gap Arrester . 25 

































ELECTRICAL SAFETY DEVICES 55 

Time-element Feature . 17 

Details of. 18 

Operation of. 18 

Tirrill Regulator. 44 

Action of. 46 

For A. C. Machine. 49 

For D. C. Machine. 44 

Tube for Enclosed Fuse. 7 










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Practical Mathematics 

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By ELMER E. BURNS and JOSEPH G. BRANCH. 



This book was writ¬ 
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One Thousand Questions and 

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By JOSEPH G. BRANCH, former Member of the 
Board of Examining Engineers of the 
City of St. lx>uis, Editor “Practical 
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By ELMER E. BURNS 

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CONTENTS 

Chapter I.—How an Electric Current Can 
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Chapter III.—Power and Efficiency of a Motor. 

Chapter IV.—Counter-electromotive force. 

Chapter V.—How Power is Lost in a Motor. 

Chapter VI.—Armatures and Cummutators. 

Chapter VII.—Types of Direct-current Motors. 

Chapter Vlll.—Starting Boxes and Their Connections. 

Chapter IX.—Curve Tracing. 

Chapter X.—How to Understand Alternating-current Motors. 

Chapter XL—Operation of Alternating Current Motors. 

Chapter XIL—Speed Control of Motors. 

Chapter XIII.—Motor Troubles and How to Cure Them. 

Chapter XIV.—Selecting and Installing Motors. 

Appendix.—Horse-power Required to Drive Various Machines 
Size 5*^x7% inches; 200 pages; fully illustrated. 

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